The traditional safety door is used either as a fire door or emergency escape exit. Two types of systems are usually used with the safety door. One is a failsafe system and the other is a non-failsafe system.
(1) The failsafe system: When the power supply is disrupted, regardless of whatever reason, the brake for the door operator is released immediately and the safety door is closed. If electric power is not disrupted in a fire, the smoke detector, the power supply will be cut off by means of a smoke detector, temperature sensor or fire detector. In other cases, devices with fusible links is molten under high temperatures and mechanically cut off the power supply to release the brake, and cause the fire door to slide down under its dead weight and close the safety door. These models are effective if power is disrupted due to fire hazards, and can stop the flames and smoke from spreading, making them highly effective in fire safety. But, if the disruption of power is not due to fire, this system will be inconvenient for the users and prevent them from using the safety door under normal circumstances.(2) The non-failsafe system: When power supply is disrupted, regardless of the reasons, the brake is still in a braking state and the safety door is not closed immediately. When a fire is detected by means of a smoke detector, temperature sensor or fire detector, a standby power of a capacitor or battery is supplied to the brake for releasing the braking state. Alternatively, devices with fusible links is melt under high temperatures, and mechanically release the brake such that the fire door slides down under its dead weight and then is closed. These models will not close the safety door immediately when electric power is disrupted. The primary advantage of this system is that if an event of power failure is not caused by a fire, it will not be inconvenient for the users and not prevent them from using the safety door under normal circumstances.
As shown in FIG. 1A, the conventional device with fusible links comprises a cord (a) connected with fusible links in series. One end of the cord (a) has potential energy, and the other end of the cord (a) is attached to a retaining device (c) which is connected to an actuating rod of a electromagnet (d). The cord (a) is stretched tight by the retaining device (c). When a fire is detected by means of the smoke detector, temperature sensor or fire detector, a standby power of a capacitor or battery is supplied to the electromagnet (d), and the electromagnet (d) actuates the retaining device (c) to release the potential energy of the cord (a). Alternatively, the fusible links on the cord (a) are molten and broken under high temperatures, and the potential energy of the cord (a) is released due to breakage of the fusible links.
Deficiencies of the failsafe type and the non-failsafe type may be improved with fusible links. However, for a person skilled in the art, it should be understood that a standby power is necessary for the device to release the potential energy. The case of the electromagnet failure, the case that the standby power is broken before a fire is detected and the case that a fire breaks out a distance away from fusible links, in anyone of them, the safety door cannot be closed. There is still room for improvement.